Leptin inhibits insulin secretion by direct effects on the pancreatic beta-cell (Kieffer et al. 1997), and thus it can modulate glucose homeostasis. Although alpha-cells and glucagon secretion also play a critical function in the control of glycaemia, the effect of leptin on these islet cells has not been examined. In the present work, we have studied whether alpha-cells are directly regulated by this hormone. The existence of several leptin receptor isoforms (a-e subtypes) was observed by PCR in the glucagon-producing alpha-TC1-9 cell line (n=3). The b isoform, the main one involved in leptin signalling, was demonstrated in alpha-TC1-9 cells by western blot (n=3) as well as in alpha-cells from OF1 mice and human by immunocytochemistry (n=3). The functional role of leptin was first analyzed by patch-clamp in both alpha-TC1-9 cells (n=8) and mouse alpha-cells (n=3). At 0.5 mM glucose, these cells exhibit electrical activity characterized by sodium and calcium action potentials (Gromada et al. 2004). In all the cells tested, leptin (6.25 nM) hyperpolarized the membrane potential, inhibiting the electrical activity induced by 0.5 mM glucose. Additionally, 6.25 nM leptin produced an inhibitory effect on the calcium signals in alpha-TC1-9 cells as well as in mouse and human alpha-cells (n=13, n=16, n=12, respectively). Similar effects on calcium signalling were observed at 0.625 nM leptin. Since alpha-cell exocytosis is calcium-dependent, we analyzed the effect of 6.25 nM leptin on glucagon secretion by radioimmunoassay. At this concentration, leptin reduced glucagon secretion at 0.5 mM glucose by almost 30% (n=8; p<0.05). However, in the presence of the PI3-kinase inhibitor wortmanine, leptin did not produce any effect, indicating that this pathway is involved. Our results indicate that leptin participates in glucose homeostasis not only via beta-cell actions but also through its inhibitory effect on alpha-cells.